Various methods for detecting knocking (knock) occurring in an internal combustion engine have been proposed. For example, a technique determines that knocking has occurred when the magnitude of vibration of the internal combustion engine is larger than a threshold value. There is a case, however, where the magnitude of such noise as vibration which occurs when an intake valve or an exhaust valve for example closes is larger than the threshold value while knocking does not occur. In this case, although knocking has not occurred, it could be erroneously determined that knocking has occurred. Accordingly, a technique has been proposed that determines whether knocking has occurred or not based on the waveform of vibration for considering such characteristics other than the magnitude as the crank angle at which vibration occurs and the damping rate.
Japanese Patent Laying-Open No. 2005-330954 discloses a knocking determination device for an internal combustion engine that uses the waveform of vibration to precisely determine whether or not knocking has occurred. The knocking determination device disclosed in Japanese Patent Laying-Open No. 2005-330954 includes a crank angle detection unit for detecting a crank angle of the internal combustion engine, a vibration detection unit for detecting a value relevant to the magnitude of vibration of the internal combustion engine, a waveform detection unit for detecting a waveform of vibration of the internal combustion engine in a predetermined range of crank angle, based on a value determined by dividing the value relevant to the magnitude of vibration by a maximum one of the detected values relevant to the magnitude of vibration, a storage unit for storing in advance a waveform of vibration of the internal combustion engine, and a determination unit for determining whether or not knocking has occurred in the internal combustion engine, based on a result of comparison between the detected waveform and the stored waveform. The determination unit determines whether or not knocking has occurred based on a value representing a deviation of the detected waveform from the stored waveform. The value representing the deviation is calculated by dividing the sum of differences which are each a difference between a magnitude in the detected waveform and a magnitude in the stored waveform determined for each crank angle, by a value determined by integrating the magnitude in the stored waveform by the crank angle.
Regarding the knocking determination device disclosed in the above-referenced publication, the crank angle detection unit detects the crank angle of the internal combustion engine, the vibration detection units detects a value relevant to the magnitude of vibration, the waveform detection unit detects the waveform of vibration of the internal combustion engine in a predetermined range of crank angle, based on the value relevant to the magnitude (intensity) of vibration. The storage unit stores in advance the waveform of vibration of the internal combustion engine, and the determination unit determines whether knocking has occurred or not in the internal combustion engine, based on the result, of comparison between the detected waveform and the stored waveform. Thus, a knock waveform model, which is a waveform of vibration when knocking occurs, is prepared through experiments or the like for example and stored in advance, and the knock waveform model and the detected waveform are compared with each other. In this way, whether or not knocking has occurred can be determined. Accordingly, whether or not any vibration of the engine is vibration due to knocking can be analyzed in more detail. Further, based on the magnitude of vibration in addition to the waveform of vibration, whether or not knocking has occurred is determined. Consequently, it can be determined precisely whether or not knocking has occurred.
The knocking determination device disclosed in Japanese Patent Laying-Open No. 2005-330954, however, detects a waveform that is normalized by dividing the value relevant to the magnitude of vibration by its maximum value. Therefore, regardless of whether the detected magnitude is large or small, the maximum magnitude of the detected waveform is “1” all the time. Thus, even if the original magnitude before being divided by the maximum value is small, the value representing the deviation of the waveform is likely to be a value which seems to represent knocking if the shape of the detected waveform is similar to the shape of the stored waveform. This is for the following reason. The value determined by integrating the magnitude in the stored waveform by the crank angle, namely the area of the stored waveform is relatively larger than the difference between the magnitude of the detected waveform and the magnitude of the stored waveform, and thus the influence of the difference between the magnitude of the detected waveform and the magnitude of the stored waveform is relatively small. Then, it could be erroneously determined that knocking has occurred, despite that knocking does not occur.